Cosmic Supremacy http://www.cosmicsupremacy.com is an on-line Space 4X game
that is free to play. It requires you to d/l a small client and then you connect to the web site thru your browser. Games are turn based and happen from every 5 mins to once an hour and there can be up to 1500 turns. Threr can be up to 64 players in a game.

It has a very good tech tree, huge galaxies, customizable ships, solar systems with up to 6 planets, 6 types of resources etc.

Premium membership allows you to setup Galaxies for play and a set of tech areas called doctrines that improve your civilization. But only premium players play againt premium players to keep it fair. Premium costs from 5 euros a month to 20 euros a year, $7 to $28.